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Mathematics at the service of the forest

Extreme temperatures have a severe impact on vegetation. The result is that, with climate change, certain species of tree are disappearing from some Swiss forests. However, mathematical models are coming to the rescue of the canopy.

In the space of 20 years, half the Scots pines of the Viège forest have disappeared. Is this due to climate change? The answer is Yes, but . . . For a long time, biologists have mainly taken an interest in the progression of average temperatures. It’s now acknowledged that it’s extreme climate situations that actually modify vegetation.

Jacques Ferrez, who works in the chair of Statistics at EPFL, and at the Swiss Federal Research Institute (WSL), is soon to publish the results of a mathematical study based on 10 years of data taken from 14 Swiss forests. “During the last 10 years, thermometers have been placed simultaneously in the forest and on the outside. We select the extreme daily temperatures, and among this data we only track unusual events. You could say that we go by the maximums of maximums and the minimums of minimums”, explains Jacques Ferrez. “In the end, statistical tools enable the comparison of the influence of various ecosystems on extremes of temperature that prevail under the canopy. Thanks to these measurements, we can monitor whether certain limiting thresholds for the survival or death of a tree have been exceeded.”

The results enable us to understand how the forest regulates the climate that prevails there. In fact, better knowledge of the influence of these extreme temperatures under the canopy helps the forest rangers and woodsmen in their decision-making. They can thus avoid keeping tree species that are in any case doomed to disappear. City planners can also refine their choice of trees when designing parks and other recreational areas.

“Scots pines, for example, cannot stand temperatures that are too high. Over the last 20 years, the number of days when the average temperature has exceeded 20°C has doubled in Viège. The tree has a certain capacity to resist these extreme temperatures, but there comes a point where – once the threshold is passed – the tree can no longer resist, and dies. This is why half the Scots pines in the Viège forest have disappeared”, adds Martine Rebetez, also of the Swiss Federal Research Institute (WSL) and guest of EPFL. “Another example concerns the Ticino region of Switzerland, where more and more palm trees are appearing. We know that the young shoots die if they are subjected to several consecutive days at – 5°C, which was the case in the past. Nowadays, the areas of Locarno and Ascona are no longer subjected to such climatic conditions, so these tropical tree species are developing freely.”

Our forests are changing – this is the reality! We can only monitor these evolutions as best we can, and these statistical studies performed at EPFL enable us to do so under the best possible conditions.